The present invention relates to a method of recovering uranium and related values, e.g., molybdenum, manganese, titanium, etc., from subterranean deposits and more particularly relates to a method of controlling calcite precipitation in an in situ leach operation by retarding or inhibiting the growth of calcite crystals in critical areas of the leaching circuit while allowing calcite precipitation in noncritical areas.
In a typical in situ leach operation for recovering uranium and/or related metal values, wells are completed into a mineral bearing formation and a lixiviant is flowed between the wells. The uranium and/or related values are dissolved into the lixiviant and are produced therewith to the surface where the pregnant lixiviant is treated to recover the desired values. For an example of such a leach operation, see U.S. patent application Ser. No. 712,404, filed on Aug. 6, 1976.
In many of the known uranium and other mineral bearing formations, a substantial part of the formation matrix is comprised of calcium-based clays (e.g., smectite) and/or limestones. When certain lixiviants (e.g., carbonate and/or bicarbonate solutions) are used in these types of formations, the calcium clays and/or limestones react with the lixiviants to form substantial amounts of calcite (i.e., calcium carbonate) in the lixiviants which, if not properly controlled, will precipitate within the leaching circuit, thereby creating scaling and/or plugging problems throughout the operation.
There are certain critical areas within the leaching circuit where this scaling or plugging is extremely serious and may jeopardize the entire operation. These areas are (1) within the production well as the pregnant lixiviant is produced to the surface, (2) across the extraction means, e.g., the ion exchange columns, used to extract the uranium and/or related values from the lixiviant, and (3) between the point where the fresh lixiviant is made and the point where it is injected into the formation at the bottom of the injection well. The continued build up of precipitated calcite in these areas will normally shut down the operation and expensive treatment procedures will be required before operation can be resumed.
As recognized in copending U.S. application Ser. No. 732,234, certain procedures may be followed to combat calcite build up in the leaching circuit. For example, excess carbon dioxide (CO.sub.2) can be added to the lixiviant at various points in the circuit which aids in keeping the calcite in solution. However, it may be difficult to keep the CO.sub.2 in solution since it easily vaporizes out of the lixiviant at the pressures existing in the leach circuit. Also, special equipment, such as calcite precipitators and/or commercially available water softeners can be used in the surface processing of the lixiviant to physically and/or chemically remove calcite. Although complete removal of the calcite from the lixiviant is most desirable, in many known leach operations the total removal of calcite from the lixiviant with such equipment would be extremely expensive and would seriously affect the over-all success of a commercial leach operation. Accordingly, there is normally some calcite still present in the lixiviant even after treatment with such equipment and this calcite can foul the extraction means used for extracting the uranium and/or related values from the lixiviant. Further, since the barren lixiviant is used to make fresh lixiviant, calcite in barren lixiviant may precipitate in the mixing tanks and/or the injection wells.
As is known, certain chemicals commonly called "inhibitors" can be added to a solution to prevent scale build up due to calcite precipitation. For example, inhibitors have been used to treat feed water for steam boilers to prevent scale in the boiler tubes. Also, inhibitors have been proposed for use in certain petroleum production operations where calcite may be a problem. However, the indiscriminate use of such inhibitors in an in situ leaching cycle where the lixiviant is to be reused would be detrimental since normal amounts of inhibitors would tend to stabilize the calcite in the lixiviant, making the removal of calcite in noncritical areas difficult and causing the calcite to build up in the recycled lixiviant to levels which would threaten the leach operation.